It is well known in the semiconductor and electronics industry to require electrical testing of circuits to ensure product quality and reliability at various stages through the manufacturing cycle. Typically, finished electronic products are comprised of passive circuit elements such as resistors and capacitors, and active semiconductor devices such as transistors and integrated circuits, all interconnected and usually physically mounted on a substrate providing mechanical support and electrical interconnection. The electrical testing is required to verify that the parts of the circuit, from individual elements to complete assemblies, demonstrate performance characteristics within design limits. For example, the conductivity of a conductive circuit trace may be tested to verify a minimum value of conductivity as well as minimum isolation from other circuit elements. Another example is the testing of resistors to measure that the actual resistance is within prescribed tolerance limits.
Since the early 1970's electronic circuits called Hybrid Circuits have been built on ceramic plates or substrates because they require very stable or high temperature operation, or high frequency operation. Passive components such as resistors are made directly on these circuits by printing and baking a partially conductive paste between the conducting traces. Typical circuits manufactured as Hybrid Circuit Boards (HCBs) include automotive engine sensors, military and aerospace components, and high frequency circuits. In the last 10-15 years with the increasing use of surface mount (SMT) components on PCBs, resistors and other passive components have been fabricated in a similar way. Today, SMT “chip” resistors are made by forming many resistors on a ceramic plate and then breaking this plate up into smaller individual components for soldering onto a PCB surface.
A Printed Circuit Board (PCB) is typically comprised of alternating layers of copper and polymer resin. The polymer resin may or may not be reinforced for improved mechanical and thermal properties by woven glass fibers or other inorganic fillers. A subset of these types of substrates is the common rigid boards used as “motherboards” in computers and cellular telephones. Another subset of PCBs is flexible circuits based on polyimide or other flexible dielectric materials, and used for interconnections requiring the ability to flex along one or more axes.
Semiconductor circuits are typically comprised of a base substrate processed as a wafer, such as silicon or other semi-conducting material, and later diced up into individual integrated circuits (ICs) called die. Circuit elements such as transistors, resistors and capacitors are built up through varied processes including layer deposition, patterning using photolithography, implantation, and metallization. Connections to the IC are typically made at surface pads which may lie near the edges of the die, or in an array cross the surface of the die as in a Flip Chip design.
Many complete electronic circuits are comprised of several of these circuit “boards” connected together or mounted on one another. For example, a computer microprocessor die may be mounted on a small PCB comparable to the size of the silicon die, and this package, termed a Chip Scale Package (CSP) is in turn mounted to the main PCB connecting to other devices. Often this small PCB is termed an “interposer” board. Another example is the soldering of high frequency hybrid circuit to a low frequency base PCB such as commonly employed in wireless devices.
In most cases, it is economically advantageous to perform electrical testing at the device or sub-component level, so that the value of rejected parts is as low as possible. Thus, as examples, electrical testing is performed during the manufacture of chip resistors that are later soldered to PCBs, CSP interposer PCBs are tested before the silicon dies are attached, and integrated circuits may be tested before dicing.
In the early 1990's some PCB manufacturers started to explore using a similar technique as that used in making HCB resistors to fabricate resistors directly on PCBs. They formed the copper traces first, and then screen printed and oven cured a paste between trace pads to form the component. At first, this technique was used to form passive components on the surface of the PCB, but by the mid 90's PCB manufacturer's realized that they could place these components on any layer including the inner layers of a multi-layer PCB. Thus during the build up of a PCB's layers, insulating and copper layers could be laminated on top of sheets with already-formed resistors or capacitors. Connections to these components are made through copper-plated holes from one layer to another in the same way that layers in a multi-layer PCB are connected. This technique is termed “Embedded Passive” (EP) technology since the elements are effectively embedded within the PCB itself.
The equipment and processes involved in forming these embedded passive components is very similar to that required for forming resistors on HCBs, although parameters for the lamination, screen printing, curing and other processes are different due to the different materials—ceramics vs. polymers—involved.
Embedded passive technology has the ability to address design challenges which require decreasing overall circuit size, increasing production yields, lowering manufacturing cost, and improving electromagnetic interference (EMI) performance of new circuit designs with higher clock frequencies. By embedding the passive components inside the PCB, up to 30% of the surface area (PCB size) can be reduced. Alternately, this real estate can be used by semiconductor chips to realise extra functionality within the same overall size. Since there is no longer a requirement to solder extremely small components, the yield for the finished PCB can increase. Since the fabrication of the embedded components is a batch process using standard PCB production techniques, the overall cost of the finished product can be reduced relative to using discrete SMT components. By embedding the components within the PCB, high frequency signals can remain shielded by outer copper layers, thereby reducing radiated fields (EMI).